The present invention relates to an optical system for an optical head, which can be used for two kinds of optical discs having different characteristics, and an objective lens for such an optical system.
There are a plurality of kinds of optical discs having different characteristics such as a thickness of a protective layer, data density and the like. For example, a CD (compact disc) or a CD-R (CD recordable) has a protective layer whose thickness is 1.20 mm. The data density of the CD or CD-R is relatively low. A DVD (digital versatile disc) has a protective layer whose thickness is 0.6 mm. The data density of the DVD is relatively high.
An optical system for an optical head used for data recording and/or readout generally includes a laser diode, an objective lens for converging the laser beam emitted by the laser diode on the optical discs, and a sensor for receiving light beam reflected by the optical discs and generating signals. In view of downsizing of the optical head, it is preferable that a single objective lens is used for various types of optical discs. Further, in order to make the optical head relatively thin, the focal length of the objective lens is preferably short.
However, when the laser beam incident on the objective lens is a parallel light, if the focal length is short, it is difficult to provide a sufficient working distance which is required to record/readout data on the optical disc having a thick protective layer, such as the CD or CD-R.
A numerical aperture (NA) is determined depending on the data density of the optical disc. For the disc having high data density, such as the DVD, it is necessary to form a relatively small beam spot. In order to form such a beam spot for a DVD, NA equals approximately 0.60. For the disc having low data density, such as the CD or CD-R, the beam spot must be larger, and the NA should be approximately 0.45. It should be noted that, if the spot size is too small in comparison with the width of a track of an optical disc, and in particular, if the employed data readout method utilizes diffraction of the light, as in data readout of the CD, sufficient diffraction effect is not provided, and therefore, the recorded data might not be readout correctly. Accordingly, when the CD or CD-R is used, the diameter of the light beam incident on the objective lens is typically confined to reduce the numerical aperture to form an appropriate beam spot (not too small) on the CD or CD-R.
In a conventional optical head, in order to vary the NA depending on the disc used, a variable aperture stop for mechanically varying the aperture size is provided at a beam incident side of the objective lens. However, such a structure requires a relatively large space, and prevents the downsizing of the optical system.